1. Field
Dental handpieces having gas-driven motors are well known. Of particular interest herein is a high-speed gas-driven dental handpiece having decreased noise and improved vibration damping characteristics.
2. State of the Art
A dental handpiece of a type having a high-speed gas-driven motor, of which the turbine rotor and bearings are replaceable as a multi-piece unit within the motor housing, is described in U.S. Pat. No. 3,074,167 to Turchi et al. A rotor of a high-speed motor of this type will rotate at speeds of 250,000 to 400,000 rpm, or more, during use of the handpiece by a dentist. Significant amounts of vibration and noise may be transmitted from the motor to the handle portion during use of a high-speed handpiece having a rotor bearing cage in direct contact with the handpiece motor housing, such as shown in the Turchi et al U.S. Pat. No. 3,074,167 patent. Vibration and noise even in small amounts can be quite disturbing to dentist and patient alike, and may be especially disturbing when, at certain critical rotor operating speeds, resonance effects greatly enhance or amplify the noise or vibration originating from the gas-driven motor.
A high-speed handpiece having reduced vibration and noise transmitting characteristics is disclosed in U.S. Pat. No. 3,499,223 to Lieb et al, wherein vibration damping rings fabricated of non-metallic or rubbery materials are interposed between the metallic rotor bearings and a metallic motor housing. These vibration damping rings are described as insertable within a gas-driven motor housing separately from the turbine rotor-and-bearing assembly. A disadvantage of this arrangement is that during removal and reinstallation of a turbine assembly by a handpiece user in a dental office one or both of the vibration damping rings may frequently become lodged in the motor housing, with resultant difficulty following in the removal of the ring. Also, reinstallation of each ring separately from the turbine-and-bearing assembly is a time-consuming chore requiring a relatively high degree of care to avoid improper orientation or seating of the ring in the motor housing.
Another problem associated with proper installation or positioning of a turbine-and-bearing assembly within a gas-driven motor housing is in obtaining proper rotor alignment and static pre-loading of the bearings within the housing. One aspect of proper rotor alignment is the degree of concentricity maintained between the motor housing axis and the axis of rotation of a dental bur which is mounted by its shank within the turbine cartridge assembly rotor shaft. The degree of concentricity is, in turn, related to bur run-out, that is, the distance of movement of a rotating bur from its theoretical axis as measured in a plane perpendicular to the rotor shaft axis, the plane intersecting the tip of the bur. Typically, a dental bur has some imbalance as a result of manufacture or because of an irregular configuration of the bur work surface. The combination of poor concentricity and bur imbalance may provide a considerable amount of vibration during use of the handpiece especially at or near the critical frequency of rotation of the turbine.
Improper axial or radial alignment of the rotor shaft with respect to the motor housing, or the application of too little or too much static pre-loading to the rotor shaft bearings, may also lead to excessive noise and vibration causing pre-mature wear and failure of the bearings. The problem of misalignment may be especially acute with a handpiece having separately-installed vibration damping rings, or spring-like bearing pre-loading elements, or both, where special care must be taken to prevent distortion of the rings, misalignment of the rotor, improper static pre-loading of the bearings, or loss of the various separately-installed components.
There is need, therefore, for a high-speed gas-driven dental handpiece having a turbine-and-bearing assembly which can be easily removed and reinstalled in a motor housing as a unitary assembly which contains all of the various parts for achieving proper radial and axial alignment to provide a high degree of concentricity, and which turbine cartridge assembly when installed in a handpiece motor housing may provide a proper degree of static preloading to the rotor shaft bearings.